1. Field of the Invention
The present invention relates generally to an optical disk recording and reproducing apparatus for recording and reproducing information on an optical disk. More specifically, the invention relates to an optical disk recording and reproducing apparatus with a focus servo system.
2. Description of the Background Art
In order to facilitate a better understanding of the features of the present invention in relation to the background art, a brief discussion will be provided about the background art of the present invention with reference to FIGS. 1 and 2, which illustrate a circuit diagram of an optical disk recording and reproducing apparatus of the prior art.
The shown optical disk recording and reproducing apparatus of FIG. 1 is designed for recording and reproducing information on tracks formed on an optical disk 1. As is well known, the optical disk is chucked on a disk drive mechanism including a motor driven spindle 2 which is driven by a spindle motor for rotatingly driving the disk. As shown in FIGS. 2 and 2A, the optical disk to be used in the shown apparatus is a so-called "pre-grooved type disc" which has a plurality of recording tracks defined by preliminarily formed grooves. In addition, as a recordable optical disk, the shown embodiment employs an optomagnetic disk. The disk 1 chucked on the spindle 2 is thus driven at a given constant speed.
An optical head 3 is provided in the vicinity of the optical disk for optically reading or writing information on the recording tracks. The optical head 3 is connected to an RF circuit 4. The RF circuit 4, operating in a reproducing mode, converts the information read from the recording track by means of the optical head 3 into an electric signal indicative of the read information to output. On the other hand, the RF circuit 4, operating in a recording mode, converts the information containing an electric signal into optical information data in a form recordable on the recording track.
This RF circuit 4 is connected to a signal processing circuit 5. This signal processing circuit 5 performs a known signal processing operation. The signal processing circuit 5 is connected to an input/output circuit 6.
The RF circuit 4 also outputs a focus error signal to a focus servo circuit 7 to feed thereto a focus error signal. The RF circuit 4 is further connected to a tracking servo circuit 8 to feed a tracking error signal. The output of the focus servo circuit 7 is connected to the optical head 3. The output of the focus servo circuit 7 is also connected to a thread servo circuit 9 which controls transverse shift of the optical head 3.
Operation of the signal processing circuit 5, the focus servo circuit 7, a tracking servo circuit 8 and a thread servo circuit 9 are connected to a CPU 10 which serves as a system controller. The CPU 10 outputs a clock signal, a timing signal, an access signal and so forth. The CPU also outputs control signals for the aforementioned respective circuits for controlling operations thereof depending upon the operation modes thereof. The CPU also serves to control the driving speed of the spindle motor for controlling rotation speed of the optical disk 1.
As shown in FIGS. 2 and 2(A), the grooves G are formed on the disk in concentric circular or helical fashion. The grooves will be hereafter referred to as "pre-grooves". Each pre-groove G has a width corresponding to where .lambda./8 (.lambda. is the wavelength of laser of the optical head). An adjacent pair of grooves G define a land which serves as a recording track T. As will be appreciated, the light intensity to be reflected from the pre-groove G and the land T is different from each other. Based on the difference of light intensity reflected from the groove and the land, a tracking error signal is generated so that a tracking servo system will control the optical head to place the light spot of the laser beam on a desired one of tracks for tracing therealong.
In such optical disk recording and reproducing apparatus, the reflected light intensity frequently varies every time the laser beam spot moves across the pre-groove during a search operation, in which the optical head is shifted transversely to the tracks. As a result, a high frequency signal St modulated by the pre-grooves, which is shown in FIG. 3(a) and will be hereafter referred to as "traverse signal", tends to be superimposed on the focus error signal.
The focus servo circuit 7 in FIG. 1 employs a phase compensation circuit 71 (FIG. 4) for enhancing a high frequency component of the focus error signal St for improving response characteristics. The output of the phase compensation circuit 71 is fed to a driver circuit 72. The driver circuit 72 generates a drive signal S.sub.FD for driving a focus actuator 73.
In this circuit arrangement, when the traverse signal superimposes on the focus error signal St, the traverse signal may be enhanced in the focus servo loop set forth above. As a result, the peak of the enhanced focus error signal tends to saturate to cause distortion of the waveform in the drive signal S.sub.FD, as shown in FIG. 3(b). This distortion of the waveform of the drive signal S.sub.FD causes variation of the direct current level. Variation of the direct current level tends to degrade the accuracy of a focusing operation of the focus servo system.